IP multimedia sub-system (IMS) is the IP multimedia sub-system in wideband code division multiple access (WCDMA) network defined by the 3rd generation partner project (3GPP) R5/R6 standard, and is a target network implementing packet voice and packet data of the 3G mobile network and providing unified multimedia service and application. The IMS adopts the IP packet domain as a bearing channel for controlling messages and media transmission, and adopts the session initiation protocol (SIP) as the call controlling message. In the IMS, the user subscription data of the IMS is gathered and managed in a home subscriber server (HSS), the service is uniformly provided by an application server (AS), and the session control is achieved by a serving-call session control function (S-CSCF), and the HSS, the AS and the S-CSCF are completely separated in the network structure. The service is triggered to the AS via the S-CSCF, and a plurality of ASs may cooperate with one another. The user gets access to the IMS via the current local agent node—proxy-call session control function (P-CSCF), the session and the service control are achieved by a home domain service node of the registration location, so that the user may obtain the same service at different access points so as to separate the service management, the session control and the bearing access, and to provide services not related to the access and the location.
As a terminal supporting multi-access mode comes forth, the multimode terminal of the user may register on the network via a plurality of access domains, and user identities registering at the same time may be the same or have a certain association (for the identities without the association, the process is the same as that of the single-mode terminal). Here, the referred multimode terminal includes, but not limited to, the multimode terminal of the WCDMA, the multimode terminal of the CDMA2000, or the mobile multimode terminal of other standards. The user may initiate or receive the call at any registration domain, or even may initiate or receive the call simultaneously at a plurality of domains.
It is a topic being researched by the 3GPP that which access network is selected by the network to be connected to the call when the multimode terminal serves as a called party. Recently, the solution proposed by the 3GPP is that an entity called as a domain selection function (DSF) is added into the IMS network. When receiving the call required to be connected to the multimode terminal, the DSF entity makes an access domain selection decision according to various factors such as the operator policy, the user preference, the terminal registration status and various elements, and connects the call to the selected access domain according to a decision result. When the DSF selects to perform the call connection in a circuit switched (CS) domain, the DSF returns a CS domain access number to the S-CSCF so as to instruct the call to be connected to the CS domain. When the DSF selects to perform the call connection in the IMS domain, the DSF returns an IMS public user identity (IMPU) of the user to the S-CSCF so as to instruct the call to be connected at the IMS domain.
During the invention process, the inventor finds that a plurality of access modes exists under a certain access domain, but in existing systems, the problem of performing the access mode selection and the call connection instruction by the network when the multimode terminal serves as the called party cannot be solved. That is, after selecting the access mode according to a certain policy, the routing decision entity cannot correspondingly instruct a routing execution entity to perform the call connection in the selected access mode.